1. Field of the Invention
The present invention relates to a method for producing semiconductor devices, more particularly to a dry etching method for organic antireflective coatings (ARC).
2. Description of Related Art
With remarkable reduction of the size of semiconductor devices in recent years, deformations and other defects in a fine resist pattern sometimes appear in a process of forming the fine resist pattern on a semiconductor substrate by photolithography due to reflection from an underlayer in the case where the underlayer under a photo-resist layer has steps or includes two or more kinds of films having different reflection factors. In order to cope with such phenomenon, an ARC is formed under the photoresist layer to appropriately control light reflected from the underlayer and thereby prevent defects in the pattern after a photolithography. After being formed, the ARC is usually patterned by a dry etching method using the fine resist pattern as a mask. However, conventional dry etching method is disadvantageous in some respects.
The conventional dry etching method for the ARC is now explained with reference to FIGS. 6(a) to 6(b).
As shown in FIG. 6(a), first, a film to be etched 22, an ARC 23 and a resist pattern 24 are formed on a silicon substrate 21. The ARC 23 is then etched with an etching gas of a mixture of a halogenated hydrocarbon containing fluorine (e.g., CHF.sub.3, CF.sub.4) and oxygen by use of the resist pattern 24 as a mask.
With the above step, however, it is difficult to maintain the width of lines of the resist pattern 24, i.e., it is difficult to control the amount of deposition on side wall of the resist pattern 24. Since the deposits are mainly generated by decomposition of the halogenated hydrocarbon, etching shift appear depending greatly on how dense the resist pattern 24 is. The amount of deposits from the halogenated hydrocarbon on the side walls of the resist pattern 24 becomes larger as the resist pattern 24 is less dense. Therefore the width of lines of the resist pattern 24 after etching tends to be larger in a less densely designed part 24 than in a more densely designed part 26 (see FIG. 6(b)).
Further, since the etching gas contains fluorine, the film to be etched 22 reduces its thickness when the ARC 23 is over-etched in the case where the film to be etched 22 is made of a silicon compound (see FIG. 6(c)). This has disadvantageous effect on the control of subsequent steps and also makes reworking for formation of the resist pattern 24 or ARC 23 difficult.
In other words, if the film to be etched 22 is over-etched during etching of the ARC 23, the film 22 becomes thin and thereby it becomes difficult to detect the etching end point of the film 22 in the later etching of the film 22 by use of the ARC as the mask. Besides, since the deposits accumulate on side walls of steps, there is a disadvantage in that the amount of etching shift greatly increases.
In order to cope with the great dependence of etching shift on the density of the resist pattern, a method has been proposed in which argon and oxygen are used as the etching gas. This method diminishes the dependence of etching shift on the density of the resist pattern. However, deposits are not generated on the side walls of the resist pattern, and therefore the width of lines of the resist pattern is greatly reduced both in the less densely designed part and in the more densely designed part.